Suppression of sprouted synapses in axolotl muscle by transplanted foreign nerves.

1. The supracoracoideus (s.c.) muscle of the axolotl shoulder is innervated by two nerves, the anterior and posterior s.c. nerves. The posterior nerve was induced to make synapses outside its normal territory in the muscle by removing a segment of the anterior nerve. Intracellular recording indicated that the efficacy of transmission from posterior nerve terminals outside their normal territory increased over several weeks prior to the return of the anterior nerve. 2. The anterior nerve reinnervated its muscle by 40-50 days after the operation, and quickly made synapses throughout the muscle. The posterior nerve territory subsequently returned to its original size and location over 3-6 months. 3. Transplantation of either of two completely foreign nerves into s.c. muscles with enlarged posterior nerve territories resulted in a similar return of the posterior nerve territory to its normal size when anterior nerve regeneration was prevented. 4. These results suggest that the advantage which newly regenerated native nerves have over sprouted foreign nerves is not the quality of 'nativeness' but rather the smaller number of synapses they support. In this view, sprouted nerves compete less effectively because they initially support more synapses per neurone than regenerating nerves.